The present invention relates generally to the field of variable area exhaust nozzles, and specifically to an exhaust nozzle flap composed of dual materials for use with turbojet engines.
The performance characteristics of today's state-of-the-art turbine engines are limited by the high temperature capabilities of their materials of construction. The strength and stiffness properties of metal alloys used in turbine engines degrade rapidly as temperatures exceed 1800.degree. F. Engines capable of performing at higher temperatures have the potential for improved specific fuel consumption and efficiency. As such, engine designers are constantly seeking higher temperature materials and design to achieve this capability. In practice, turbines with high temperature components now resort to the use of air cooling and complex (and therefore costly) hardware designs to keep the materials of construction within acceptable temperature limits. The diversion of cooling air from the engine cycle to achieve this however, acts to reduce overall engine efficiency. Therein lies the challenge--the application of a new family of materials that can operate at extreme temperatures without cooling air.
Carbon-carbon composites have the potential for providing air-breathing engine designers a lightweight (low density), high temperature, high strength-to-weight material for a number of applications. Carbon-carbon composites maintain strength and stiffness properties up to approximately 4000.degree. F. In short-life engines, such as a cruise-missile engine, carbon-carbon has the potential to provide a high performance payoff and may find application in very highly stressed components such as turbine disks or bladed disks (blisks). Other components for short-life engines include nozzle vanes, combustor liners, turbine shrouds, and perhaps, even shafts.
Another class of engine--the long life engine for the tactical fighter--can benefit from carbon-carbon composite materials. Most augmented turbofan engines are equiped with outer nozzle flaps to control the airflow over the rear of the engine installation. However, these turbojet engines have exhaust gas temperatures above the limit of practical heat resistant metallic materials. The only available coolant for cooling such exhaust nozzle parts as the flaps is compressor air. Compressor bleed is a highly undesirable source of coolant because of the resulting reduction in engine efficiency and thus aircraft range. Exhaust nozzles constructed of materials which will operate above the limits of metals and will tolerate the extreme exhaust gas temperatures of modern turbojet engines without need of cooling provisions would be of great benefit to engine efficiency.
The task of providing an exhaust nozzle flap which is capable of resisting the extreme exhaust gas temperatures of turbojet engine is alleviated, to some degree, by the following U.S. Patents, which are incorporated herein by reference:
U.S. Pat. No. 2,548,485 issued to I. Lubbock on Apr. 10, 1951; PA1 U.S. Pat. No. 2,926,489 issued to F. Halford et al on Mar. 1, 1960: PA1 U.S. Pat. No. 3,943,703 issued to S. Kronogard on Mar. 16, 1976; and PA1 U.S. Pat. No. 4,201,611 issued to E. Stover on May 6, 1980.
The disclosure of Halford et al illustrates a prior art adjustable propulsion nozzle which, when composed of a single material, requires a means of cooling. In the disclosed device, cooling air is supplied by pipes and passages formed in the housing of the propulsion nozzle.
The Lubbock reference illustrates the use of a two-part combination of materials in a combustion chamber, including a metal for strength and a ceramic for heat resistance. However, in Lubbock, the lining is attached to ribs which are welded or otherwise fixed to the wall. A problem with fixing any lining to a support composed of a different material is that the different thermal expansion characteristics result in severe thermal stress conditions. One solution to the inequality of thermal expansion characteristics is supplied by the Kronogard reference. In Kronogard, slotted clamping members make thermal movement possible between a ceramic material and metal in a gas turbine power plant.
An alternative to conventional ceramics are carbon-carbon fabrications. The Stover reference discloses a method of fabricating compositions composed of carbon-carbon fiber composite materials.
Materials such as an oxidation protected composite of graphite in a carbon matrix (carbon-carbon) or siliconized silicon carbide, a ceramic type material, are available which will tolerate the extreme temperatures in question. It is difficult to construct highly loaded structural nozzle parts such as the nozzle flaps of these materials for two reasons:
(1) The characteristics of low ductility of ceramic materials and of the oxidation protecting ceramic case used on carbon-carbon materials render them unsatisfactory for use in nozzle flap elements such as cams, inter flap seal retention and positioning fittings and compact, thin wall hinges and lugs for concentrated load application.
(2) It is difficult to fabricate light weight flap structures because of the impracticability of providing high section modulus structural members (I and box sections) and intersecting rib configurations in composite construction. This difficulty tends to lead to low structural efficiency, thick sections and excessive weight in general. This difficulty also tends to inhibit the realization of the light weight design which would be expected from a high strength to density ratio material such as carbon-carbon composite.
In view of the foregoing discussion, it is apparent that there exists a need for an exhaust nozzle flap design which takes advantage of the heat resistant properties of carbon-carbon and siliconized carbide materials, and accomodates the relative thermal expansion between the heat-resistant layer, and a stiffer structural support layer. The present invention is directed towards satisfying that need.